You will need to attack the alien Ceph aggressively so they fire on you rather than your four NPC allies. Human soldiers die very easily, so you need to attack aggressively to keep your allies alive. You may want to play this mission on the easiest difficulty (and with armor modules) to make the attempt as easy as possible.

Mark the four friendly soldiers (including Chino, although he never dies) once you start the mission. Those are the four marines you need to keep alive for the engagement. All the other marines can die and you will not be penalized. IGN found it handy to let the Delta squad marine at the HMG die so you can use the weapon yourself.

On the easiest setting, you can switch to armor mode and snipe/plink the Ceph who attack. Stand on the yellow container where you detached the HMG -- Chino's marines will gather there, and the vantage point you get lets you shoot all the enemies who charge your position at the trench. The condition is satisfied once you fight past the last wave of Ceph and enter the building marked by Chino.